1. Field of the Invention
This invention relates to printed circuit cards and usage of variable width signal traces and spacing to maintain constant differential impedance.
2. Description of Related Art
Traditional printed circuit board (PCB) design uses a fixed trace width for signal routing. For any given signal trace width, fixed spacing is required between a differential pair to achieve a constant differential impedance. The fixed trace width is limited by the spacing between pins and must remain constant even after the signal trace leaves the package pin field.
Modern high speed communication interconnects often use differential signals to transmit signals over long distances. Differential signals reference each other rather than a reference plane voltage, such as a ground plane voltage. The differential signals can be transmitted across a long distance without a common ground reference between the source and destination system. A source system delivers a pair of signals which, ideally, are exact opposite states. At the receiving end, the difference between the two signals is evaluated and the correct state of the signal is determined. The amplitude of the signal transmitted is determined by the differential impedance of the interconnect. For a fixed amount of driver current, the higher the differential impedance, the larger the signal swing which is desirable for signal communication.
FIGS. 1A and 1B are cross-sectional views of a conventional microstrip and a conventional stripline, respectively, in a PCB 100. The PCB has a dielectric or insulator material 120 with a thickness h. Transmission lines on PCBs 100 can have two or more conducting paths: two conductors (also known as traces) and/or a conducting plane in close proximity to a conductor. The conductors can be in the form of a microstrip transmission line 105 (see FIG. 1A) or a stripline 110 (see FIG. 1B). Hereafter, xe2x80x9cmicrostrip transmission linexe2x80x9d is simply referred to as xe2x80x9cmicrostripxe2x80x9d for convenience. Both types of conductors have reference image planes, sometimes called virtual-ground planes, which may be either circuit ground or power planes 115. As seen in FIG. 1A, a microstrip 105 has a surface conductor separated from a reference plane 115 by the dielectric material 120. As seen in FIG. 1B, a stripline conductor 110 is embedded in the dielectric and located, centered or otherwise, between two conducting reference planes 115.
In both FIGS. 1A and 1B, the conductor width w specifies the width of each trace. Conductor thickness t specifies the thickness of each signal trace and conductor spacing s specifies the distance between the inner edges of each trace. The dielectric thickness h specifies the thickness of the dielectric measured from the PCB reference groundplane 115 to the bottom of the trace.
The dielectric layer in these structures is described by a dielectric constant ∈r relative to that of free space. The dielectric constant of free space is equal to one (∈r=1). The dielectric constant for a microstrip is a combination of the dielectric constants of air above the lines and the board insulator material 120 below the lines. The effective dielectric constant for a microstrip is equal to the dielectric constant of the base material. The effective dielectric constant for a stripline is determined by the dielectric 120 embedding the conductor.
Differential transmission lines are made of two strip conductors spaced closely and forming a complete conducting loop path for the signal. A conducting plane is not needed to form a complete transmission path.
A need exists for signal traces to maintain constant differential impedance while allowing the signal traces to escape tight package pin pitch and maintain relatively low DC loss.
Flexible use of different signal trace widths and spacings controls differential signal trace impedance on PCBs. Differential impedance of a signal pair is determined by the geometry and spacing of individual traces. The value of the differential impedance is inversely proportional to the width and directly proportional to the spacing of the traces. By decreasing or increasing the trace width and spacing simultaneously, a constant differential impedance can be achieved. Methods and apparatus for microstrips and striplines are directed to using variable width signal traces and spacing to maintain constant differential impedance while allowing signal traces to escape tight package pin pitch and maintain relatively low DC loss.
In accordance with an embodiment of the invention, a method of controlling differential impedance using variable trace width and spacing includes selecting a differential impedance to be maintained on the circuit; constructing a constant differential impedance plot based on an impedance model, signal trace width and signal trace spacing; selecting a maximum signal trace width and spacing from the plot for a package pin field; and selecting a signal trace width and spacing from the plot for an area outside the package pin field such that differential impedance remains constant.
In accordance with an embodiment of the invention, an apparatus for maintaining constant differential impedance on a circuit includes a printed circuit board with a dielectric material of a constant thickness; at least one package pin field on the printed circuit board; and a microstrip including a pair of conductors of constant thickness on the printed circuit board wherein differential impedance along the length of the pair is constant.
In accordance with an embodiment of the invention, an apparatus for maintaining constant differential impedance on a printed circuit board includes a printed circuit board with a dielectric material of a constant thickness; at least one package pin field on the printed circuit board; and a stripline including a pair of conductors of constant thickness inside the printed circuit board wherein differential impedance along the length of the pair is constant.